Neogonodactlyus bredini with damage on its predatory appendage from another mantis shrimp's strikes! Photo by Roy Caldwell.

Ask most anyone what butterflies use their wings for or what fish do with their fins and you will undoubtedly hear an answer like, "Wings are used for flying and fins are used for swimming!" Some body parts just seem so well-adapted to perform certain functions; this is why there is a paradigm in biology that "specialized" body parts correspond to specific ways in which animals go about their daily business. In other words, specialization in morphology corresponds to specialization in ecology. A classic example of this concept is variation in the beaks of the Galapagos finches. Some finches have beaks adapted to crush hard seeds, while others have beaks specialized for eating insects.

However, not all animals seem to exhibit this pattern. The marine crustacean known as the mantis shrimp has legs, called predatory or raptorial appendages, which can produce one of the fastest movements in the animal kingdom. These raptorial appendages come in many shapes ranging from sharp spear-like appendages to hammer-like appendages. Mantis shrimp use their fast-moving appendages to crush open snails and other hard-shelled marine organisms, so they can eat the soft bodies inside. However, mantis shrimp also appear to eat other foods, like fish, which probably do not need to be smashed to bits before they are consumed. Even though they have specialized legs well adapted to smashing or spearing prey, some species may not use their raptorial appendages for this purpose. The goal of my research is to determine if mantis shrimp have diverse diets. Then if so, I will see how diet diversity correlates with raptorial appendage morphology across the mantis shrimp family.

First, a little background about mantis shrimp. Mantis shrimp are closely related to decapods, such as lobsters, crabs, and true shrimp. Even though mantis shrimp look like decapods, they actually branched off and became their own group 400 million years ago. Mantis shrimp have the most complex visual system ever reported in the animal kingdom. They are also one of the fastest swimmers in the sea, swimming at speeds of up to 30 body lengths per second — comparable to speeds measured in squid, which previously held the record.

But my favorite characteristic of mantis shrimp is of course their lightning fast raptorial appendages. Researchers in the Patek Lab at the University of Massachusetts and Caldwell Lab at Berkeley have found that a mantis shrimp’s predatory strike can move 23 meters per second (50 miles per hour) and produces accelerations that are comparable to a flying bullet! So it would be surprising if some mantis shrimp species were capable of this rapid movement, but didn't use it to catch prey. Hence, my study of mantis shrimp diets! I am using two techniques, stable isotopes and behavioral studies, to figure out which food items mantis shrimp eat.

Before I could study their diets, I first had to collect several different species of mantis shrimp and their possible prey. Most mantis shrimp live in the tropics, so I have traveled to Lizard Island, Australia and Mo’orea, French Polynesia to collect the animals. However, my main field site is in Colon, Panama where I collect at the Smithsonian Tropical Research Institute’s Galeta Marine Laboratory. After collecting, I transport all of the specimens back to the UC Berkeley Center for Stable Isotope Biogeochemistry, where I analyze the carbon and nitrogen stable isotopes of mantis shrimp and their prey.

What is a stable isotope? Let's go back to high school chemistry for a moment! A normal atom has the same number of neutrons and protons in the nucleus, but a stable isotope has more neutrons than protons in the nucleus. For example, a normal carbon atom has 12 neutrons in the nucleus, but its stable isotope has 13 neutrons. These isotopes are stable, because they do not exhibit radioactive decay over time — they won't lose that extra neutron — which means that the isotope will always have 13 neutrons in the nucleus. Researchers look at the ratio of normal atoms to stable isotopes to track diet, because the ratio of normal atoms to stable isotopes in the body of a predator can reflect the type of prey it has eaten. For example, if the mantis shrimp has a ratio of 10 carbon-13 atoms to carbon-12 atoms and the crab that you think the mantis shrimp eats has a ratio of 8, then there is a good chance that the mantis shrimp eats this species of crab. The reason why the mantis shrimp’s ratio is not exactly 8 is that there is an expected change in the predator’s ratio that occurs when the predator metabolizes the prey. You are what you eat (plus a little bit!), and stable isotopes allow us to track this pretty accurately.

Back In the laboratory, my assistants and I identify all of the prey items and stomatopods that we collected. We then take muscle tissue samples from the mantis shrimp and from the prey. We use a mass spectrometer to analyze the carbon and nitrogen stable isotopes in both the mantis shrimp and prey tissue. Finally, we compare the isotope ratios of the mantis shrimp and prey to determine who ate what. Since the mantis shrimp is what it eats, all prey items that have isotope ratios similar to the mantis shrimp’s ratios are likely a part of the mantis shrimp diet.

To confirm the accuracy of the stable isotope analyses, I also conduct behavior experiments that help me to determine which animals mantis shrimp are physically capable of eating. To do this, I stock aquaria with mantis shrimp and potential prey, and I wait to see which prey the mantis shrimp eat. So far, I have performed this experiment on only one species, but eventually I will look at many species of mantis shrimp, with different appendage morphologies, to see if mantis shrimp with different appendage shapes have different diets. Together with the isotope analyses, these experiments will give me a good picture of mantis shrimp diet and ultimately lead to an in-depth understanding of the relationship between raptorial appendage morphology and diet across the mantis shrimp family. This fall, I’ll return to Panama to complete my field experiments, so stay tuned for updates in future blog posts!

Sometimes, the study of basic biology can lead to technological advances, and a recent discovery about the vision of mantis shrimp is a perfect example, providing insight that could help us improve the technology inside DVD players. What is the connection? Circularly polarized light!

You're probably familiar with linearly polarized light. Fishermen often wear polarized sunglasses to reduce the glare from the water and make it easier to see the fish. Typically a ray of light vibrates randomly in all planes, referred to as e-vectors. When light reflects off water at a certain angle, only waves with certain e-vectors are reflected. A linear polarizing filter can be oriented to block those waves, allowing us to see the rest of the light that has passed through the water and is reflected by the fish below. But light can also be circularly polarized, travelling like a corkscrew, twisting either clockwise or counter-clockwise. We can’t see this property of light, but there is one animal that can!

Odontodactylus scyllarus is a stomatopod, or mantis shrimp, living in the Great Barrier Reef. Stomatopods have the most complex eyes in the animal kingdom. About a year ago, UCMP Director and Faculty Curator Roy Caldwellwas part of a team of scientists who discovered that when light bounces off the hard exoskeleton of some stomatopods, that light is circularly polarized. What was particularly surprising was that the stomatopods responded to that light — they were capable of seeing circularly polarized light! What eluded Roy and others was how.

Now colleagues have discovered that the stomatopods don't see the circularly polarized light directly. Special photoreceptor cells in their eyes, called R8 cells, filter/convert the circularly polarized light into linearly polarized light, which can then be sensed by other photoreceptor cells below it. The R8 cell is quite remarkable and might serve as a model for tiny manmade dual-function microsensors.

Manmade filters that convert polarized light are called quarter-wave retarders and are effective only across a very narrow band of wavelengths. The R8 cell (acting like a quarter-wave retarder) can filter light across a wide band of wavelengths, spanning the entire visual spectrum, into the UV spectrum.

There are lots of applications for a highly effective quarter-wave retarder, including DVD players. As DVD technology advances, people are already using circularly polarized light to create 3D movies  one eye sees the clockwise corkscrews of light, and the other eye sees the counter-clockwise corkscrews (Roy received some prototype 3D glasses using this technology and used them to verify that the stomatopods were producing circularly polarized signals!). Digital cameras along with many other optical devices also include quarter-wave retarders in their sensors.

We can learn a lot about optics from the stomatopod eye, and apply this knowledge to new technologies.